Effects of Soil Moisture on Shrub Seedling Survival in Semi‐Arid Grassland

Abstract

Grazing of wooded grasslands by domestic stock has led to dominance by woody plants, in many parts of the world. In semi—arid grasslands in Australia Dodonaea attenuata is a common, invasive shrub of perennial grasslands on sandy soils. There is evidence that cohorts of D. attenuata establish infrequently in vigorous grasslands and more frequently where the perennial grass has been damaged by overgrazing. This study examined the influence of herbaceous growth on mortality of D. attenuata seedlings, by growing them in plots with controlled moisture input. The herbaceous layer in the plots was either natural, clipped, or killed. Without summer irrigation all the seedlings in the natural grassland died. Seedling survival was inversely related to the amount of herbaceous growth on the other treatments. Summer irrigation maintained D. attenuata seedlings and produced a large biomass of perennial grass, which left the seedlings susceptible to grass fires. A water—balance model was run for the experimental site for the 97 yr of climatic records. The soil—moisture patterns associated with the establishment of three known cohorts of D. attenuata were identified. The model output was interrogated for similar soil moisture patterns in other years. It was estimated that only six widespread D. attenuata establishment events have occurred in 97 yr. In intact grasslands D. attenuata rarely establishes densely, and when it does so it is susceptible to grass fires. Modern management has increased establishment frequency by increasing the availability of summer soil moisture by overgrazing the grasslands and has suppressed grass fires. This has changed open grassland to dense shrubland.

Impacts of temperature extremes on European vegetation during the growing season

“…The shrub foliage can be either evergreen or deciduous" (Strahler et al, 1999, p. 17). This sparse canopy cover leaves the soil prone to a loss of moisture during high temperature events, which may in the course damage non-perennial grass seedlings that heavily de-pend on moist conditions in the top soil layer (Harrington, 1991). While this phenomenon can already be observed in spring in these regions, it appears throughout the season and seems to reach its peak in the time period May-June.…”

Altered water and nitrogen input shifts succession in a southern California coastal sage community

“…In many areas, native shrubs have encroached into grasslands, apparently due to increased livestock grazing and decreased fire frequency (Harrington 1991, Brown and Archer 1999, Van Auken 2000. In Californian drought-deciduous coastal sage scrub (CSS or ''soft chaparral'') and in other mediterranean-climate shrublands around the globe, the opposite trend is observed, with a frequent conversion of native shrublands to systems dominated by a few invasive species, such as conversion of CSS to nonnative Eurasian grasslands (Stylinski and Allen 1999, Cione et al 2002, Vila et al 2003, Gaertner et al 2009).…”

Shade, nutrients, and grass competition are important for tree sapling establishment in a humid savanna

“…In many savannas, seeds germinate with the onset of rainy season after being stimulated by dry season fires (Gashaw and Michelsen 2002). However, an early wet season drought after germination can cause heavy mortality of tree saplings (Harrington 1991, Hoffmann 1996, Gignoux et al 2009). Therefore, frequent rainfall is more important than overall mean annual precipitation in estimating tree sapling establishment (Hoffmann 1996, Wilson and Witkowski 1998, Kraaij and Ward 2006.…”